Methods for Reduction of Voltage Unbalance in Weak Grids Connected to Wind Plants

[attachment=16994]

1 INTRODUCTION
The location of wind turbines is based on wind properties. Normally, areas with good wind resources are geographical remote. The cost of the electrical power grid connecting the wind plants should be as low as possible. The voltage quality in a weak power grid may be poor, and unbalanced voltages can occur. Induction machines are attractive as wind turbine generators due to their low cost, ruggedness and the need for little or no maintenance. If an induction machine is connected to even a low unbalanced voltage, its stator currents will be highly unbalanced. The unbalanced current creates unequal heating on the stator winding and torque pulsations in the generator.


FROM AC-QUANTITIES TO DC-QUANTITIES
To eliminate the need for three phase symmetry, a single-phase synchronous frame transform is used. The single-phase synchronous frame transform was introduced in [1]. Imagine the phase voltage Va, shown in Fig.2, rotates around the origin by a speed of 60Hz. Then let the dq frame also rotate with 60Hz around its origin. The quantities Vqa and Vda are dc values and can be calculated by the angle φa and the absolute value |Va|. The synchronously rotating qd-axes for phase A are shown in Fig.2 and the transform is described in the following text.

5 SIMULATION MODELS
A power system model, similar to the illustration in Fig.3, was implemented in PSCAD/EMTDC. The phase voltages at the source can be regulated independently. An unbalanced load voltage is created by decrease the phase A voltage at the source.